The present invention relates to switching-mode power supplies (SMPS), and more particularly to a startup circuit that is configured to produce a low startup current for switching-mode power supplies.
Regulated power supplies are indispensable in modern electronics. For example, desktop and laptop computers often receive unregulated power input from various outlets and provide regulated power supplies on the motherboard to the CPU, memories, and periphery circuitry. Regulated power supplies may be used in a wide variety of consumer applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
A switching-mode power supply (SMPS) is an electronic power supply unit that may be regulated using a switching-mode controller. In general, a switching-mode controller rapidly switches a power transistor between on and off states with a variable duty cycle (pulse width modulation, or PWM) or variable frequency (pulse frequency modulation, or PFM), whose average is the desired output voltage. In a PWM controlled power supply, the duty cycle of the control pulse may be used to control the output of the power supply, whereas for a PFM controlled switching-mode power supply, the switching frequency may be controlled in response to load conditions.
SMPSs may have many advantages such as smaller size, higher efficiency, and larger output power capability, and are widely utilized in mobile phone chargers, notebook computer adapters, and other fields. In recent years, with the advent of green technology, companies are motivated to develop green power supplies with improved energy efficiency and low standby power consumption.
FIG. 1A is a simplified schematic diagram of a switching-mode power supply (SMPS) system 100 having a switching-mode controller 120 that includes a conventional startup circuit 130, which is described in more detail below in connection with FIG. 1B. SMPS system 100 also includes a full-wave bridge rectifier 106, a capacitor 104, and a transformer with a primary winding 103, a secondary winding 114, and an auxiliary winding 115. Rectifier 106 and capacitor 104 convert AC input Vac to a rectified DC line voltage VIN. SMPS 100 also has a power transistor 107 coupled to controller 120 for controlling the on/off of current flow in primary winding 103, and a resistor 108 for sensing the current. Resistors 109 and 110 are connected to auxiliary winding 115 for providing a feedback signal FB to controller 120. SMPS 100 also has a resistor 116, a capacitor 101, and a diode 105 for providing a power supply to the Vcc terminal of controller 120.
Controller 120 has a startup threshold voltage Vst, which is used to determine whether the system is in a startup mode or a normal operation mode. When the system is in the startup mode, Vcc is below Vst, and controller 120 does not provide switching signals to power transistor 107. As a result, no energy is delivered from auxiliary winding 115. As depicted in FIG. 1, during the startup mode of system 100, capacitor 101 (also referred to as Cst) is charged by rectified line voltage VIN through startup resistor 116 (also referred to as Rst). The startup time (also referred to as Tst) for system 100 may be calculated using the following equation:Tst=(Cst×Vst)/(Is−I2); with Is=(VIN−VCC)/Rst, where Vst is the startup voltage or threshold voltage for system 100 and I2 is the startup current for controller 120. According to the above equation, for any given values of line voltage VIN and startup resistor 116, the startup time for system 100 may be reduced by reducing the startup current (I2) for controller 120.
When Vcc reaches Vst, the system enters the normal operation mode. During the normal operation mode of system 100, the OUT pin of controller 120 may provide one or more switching signals to turn power transistor 107 on and off to regulate the output voltage of secondary winding 114. Since controller 120 may draw more current during the normal operation mode of system 100 than during the startup mode, capacitor 101 may receive energy from auxiliary winding 115 to supply the operating current to controller 120 during the normal operation of system 100.
FIG. 1B is a simplified schematic diagram of conventional startup circuit 130 for a switching-mode power supply such as system 100 of FIG. 1. As depicted in FIG. 1B, an internal bandgap reference circuit 132 produces a reference voltage Vbg, and a regulation circuit 135 produces an internal power supply voltage Vdd. Here, supply voltage Vcc is sampled using a resistor divider circuit including resistors 141, 142, and 143. The sampled value of Vcc is compared with Vbg at a comparator 144. The voltage value for threshold voltage Vst may be determined based on the voltage drop across resistors 142 and 143 and reference voltage Vbg. When the voltage drop across resistors 142 and 143 exceeds the voltage value for Vbg, comparator 144 outputs an Enable signal, indicating that the switching-mode power supply controller has entered the normal operation mode. Conversely, when the sampled valued of Vcc drops below Vbg, the system goes in the startup mode. Additionally, a switch 146 in FIG. 1B may be used for functions related to hysteresis.
As depicted in FIG. 1A, the current flowing through resistor 116 includes the charging current for capacitor 101 (I1) and the startup current for controller 120 (I2). A small value for resistor 116 may generate a large current, which in turn shortens the startup time. However, a large current consumes more energy, thereby increasing system standby power. On the other hand, the value for capacitor 101 may not be reduced due to the controller normal operation current restriction.
From the above, it is seen that even though switching mode power supplies (SMPS) are widely used, they suffer from many limitations. Therefore, improved techniques for reducing the startup current (thereby reducing the standby power) while shortening the startup time for switching mode power supplies is needed.